In recent years, according to downsizing and weight saving of portable information terminals, it becomes possible to carry them easily and consequently they have been used among many users. Accordingly, many researches are conducted for constructing a wireless ad-hoc network as an on-demand type of communication, in order to freely exchanging information under a mobile environment. This wireless ad-hoc network is intended to provide communication means to send and receive data among terminals that have temporarily aggregated, as a form of mobile computing, in the context where distance and time get close; i.e., it is the network that is configured on demand by people having information terminals.
In this wireless ad-hoc network, there has been suggested a communication scheme that specifies as a destination not a specific terminal but a party that is based on the position of the recipient, time, or the user's matter of concern. As a physical layer supporting this, the broadcast relaying transmission is considered, which does not require infrastructure facilities such as a base station in the existing mobile communications, but employs the short-range wireless communication having the coverage of several tens of meters at the most. Also, as a situation where this communication is carried out, involuntary information exchanges between pedestrians or between a pedestrian and a shop, and a communication that is based on the same matter of things on the commuter train, etc., are assumed. Under such communication environments, since it is difficult to intensively manage the position information and the path information of terminals as a network, it is supposed that all communications are implemented as broadcast to the entire network. Note that broadcast means that the destination of data shall not be specified and data are to be sent to an unspecified number of partners.
On the other hand, as an existing communication scheme, there is multicast that sends the same data to a plurality of specified destinations. In the multicast, a packet is selectively relayed so that it is expected that communication transmission may be possible with a smaller amount of communication than broadcast. Among multicast communication schemes, ODMRP (On Demand Multicast Routing Protocol) is cited as what can best correspond to an ad-hoc environment and the change of topology of the network.
In the ODMRP, a terminal that transmits to the group regularly broadcasts a member information of the group to the entire network. When a receiving terminal participating in this group receives this member information, it updates the member table it owns and sends back the member table to the sender via the opposite path against the broadcast. With these information exchanges, information about members and paths is always updated among all members. In this ODMRP, many problems when applying multicast to an ad-hoc network are solved by reason that a specific server is not required, and a multicast group is first generated automatically when a packet exists, etc. Also it is advantageous that an amount of communication is held to a minimum when an amount of communication in the group is much greater than frequencies of broadcast of member information.
On the other hand, using broadcast as the lower layer in the communication with a party sharing such a topic, information is screened only by filtering at a receiving terminal. Even in the scheme using broadcast for the lower layer, it is possible, in case of a uni-directional communication such as an advertisement, to restrain an increase of packets by limiting the frequency of the transmission of information. However, in case of bi-directional communications such as when providing information from a peripheral terminal in response to a query from a terminal or exchanging information among people sharing a hobby, congestion may easily occur and lead to a traffic explosion. Further, each portable terminal would use radio wave resources more than necessary because of retransmission to a terminal having no interest in the information, so that valuable electric power and information processing capacity would be wasted. Such a tendency is particularly eminent where the scale of network becomes greater, and the ratio of participants in a party in the network is smaller. Therefore, it is practically doubtful to establish a bi-directional communication with a party by relying entirely on broadcast.
Seen from another standpoint, when establishing a communication with a party by means of multicast communication such as the ODMRP, there is a trouble where the transmission of control information for the group management occurs overheads. In particular, in a case where a couple of replies are expected at most in response to a query, the broadcast for control information would need more communication than actual information. Further, in a case where terminals are moving actively, a situation where the path has already been nullified would occur when attempting to send a packet to the group. Therefore, a management method, such as a conventional multicast communication, which assumes a group that will persistently continue to exist, is not applicable to the communication among the group such as pedestrians who go through a road, where the active movement occurs.
In order to solve the above-mentioned problems, it is an object of the present invention to enable, in an on-demand type of communication where the active movement occurs, the bi-directional communication among a party composed of an unspecified number of terminals that share a topic. It is another object of the invention to dramatically reduce a traffic in the network among the same group, as well as to reduce power consumption of communication terminals. It is a further object of the invention to continue communication without a break among the remaining terminals even if a given communication terminal moves out of communication range.